This study examines obese and lean individuals on two standard diets (which differ by calorie content) and the stool and urine energy losses to compare these values in lean versus obese individuals. The study is also investigating the specific macronutrient content of stool to examine differences in these parameters between lean and obese individuals. The repeatability of these measurements is also being tested in a subset of individuals. In collaboration with Dr. Jeffrey Gordons lab at Washington University in St. Louis, samples of stool will be sent for determination of the fecal bacterial flora phylotypes to examine the association between stool energy loss and enteric flora populations. Preliminary results from an initial 14 lean and 9 obese individuals failed to show any difference in stool or urine energy loss. The range of calories stool was large, varying from 2 to 9 % in both groups. Stool calorie loss on each diet was associated in each individual. On the higher calorie diet, obese individuals had greater stool calorie loss (so absorbed less food ) than lean. Preliminary results show acute changes in gut bacteria with nutrient load.